1. Field of the Invention
The present invention relates to a disk drive servo system for compensating for eccentricity of a disk in a disk drive, and more particularly, to a disk drive servo system for eccentricity compensation using a compensation table including compensation control values which correspond to rotational speeds of a disk drive and are obtained through repeated control learning processes and frequency response characteristics of a disk drive turntable actuator, and a method of compensating for a disk drive eccentricity.
2. Description of the Related Art
FIG. 1 shows an example of a tracking error signal due to eccentricity in an optical disk drive servo system. The eccentricity in the optical disk drive servo system occurs when a rotational axis of a spindle of a disk drive rotating a disk deviates from a track center of the disk. Since the eccentricity is a main periodic disturbance component and may occur in every period 1T of the spindle, as the rotational speed of the disk drive increases, the influence of the eccentricity on a tracking error signal TES also increases. If the eccentricity is not compensated, an accurate tracking operation cannot be performed. Accordingly, a conventional system has employed various methods of compensating for the eccentricity.
FIG. 2 is a block diagram of a pickup head (PUH) position control system of a general optical recording and reproducing apparatus. The PUH position control system receives a nominal position signal indicating a desirable position of a PUH as an input signal. A feedback signal indicating an actual position of an actuator 220 moving the PUH in an optical disk drive and representing a change of a desired position is fed back to an adder 200 where it is added to the nominal position signal. An error signal “e” output from the adder 200 is input to a controller 210. The controller 210 performs a predetermined algorithm of compensating for the error signal “e” and applies a compensated control output signal to the actuator 220. The actuator 220 moves the PUH in response to the compensated control output signal received from the controller 210. Such operations are repeated to adjust the position of the PUH; however, a large eccentricity cannot be overcome with such a conventional control system.
FIG. 3 shows an example of another conventional technique in which a procedure of performing eccentricity compensation is added to the control system shown in FIG. 2. In the system of FIG. 3, it is assumed that an eccentricity signal is of periodic nature and has a sinusoidal form, Asin(ωt+φ), of amplitude A, a disc rotational frequency ω and phase φ. A method of compensating for the eccentricity is disclosed in U.S. Pat. No. 5,892,742. In the operation of the system of FIG. 3, before the tracking control starts, a feedforward control value 300 is calculated from an error waveform (TES) as shown in FIG. 1. In other words, an amplitude of the eccentricity is determined using the number of track errors occurring during one track rotation period as shown in FIG. 1, and a phase of the eccentricity is determined based on a delay time between an actuator drive spindle index reference signal indicating one rotation and a track error having a maximum amplitude. The calculated feedforward control value 300 is added to the output of the controller 210. This eccentricity compensation method is simply embodied and easily applied to the control system. However, since this method is a sort of an open loop method which does not consider response characteristics of a servo (actuator) control system, and since the periodic eccentricity does not fully approximate to a complete sine wave, a performance of the control system is limited.
FIG. 4 shows another example of a conventional technique in which another procedure of performing the eccentricity compensation is added to the control system shown in FIG. 2. A control system of FIG. 4 is disclosed in U.S. Pat. No. 5,550,685 and is applied to a hard disk drive system. In this control system, before a control process starts, a fixed feedforward control value is obtained using a track error signal and is then stored in a table 400, and during the control process, an error due to the eccentricity is compensated using the stored feedforward control value.
In addition, considering changes in the Repeatable RunOut (RRO) characteristics occurring due to external factors during an operation of the disk drive system, an adaptive feedforward controller 410 is additionally provided. For this, a discrete Fourier transform (DFT) is performed to extract a particular frequency component from a position error signal (PES or tracking error signal TES) “e”, and then an inverse DFT (IDFT) is performed to obtain a correction signal of the particular frequency component. The correction signal of the particular frequency component is added to the PES “e”, and the result of this addition of the correction signal and the PES “e” is added to an error input of an existing servo control loop. Through these operations, an error compensation control process can be performed. The control system of FIG. 4 is embodied considering the response characteristics of an entire closed-loop. Although it is more complex than the previous one, it is more effective. However, the control system of FIG. 4 does not consider eccentricity components at various frequencies but at a particular frequency only. In addition, the control system of FIG. 4 cannot adapt itself to a change in the rotational speed of the disk drive.
While the amplitude of the eccentricity remains constant, the frequency component of the eccentricity varies with a playback speed of the disk drive. Since the playback speed of the disk drive changes according to the frequency response characteristics of the actuator of the disk drive, the eccentricity has different influences on the control system, so it is required to change a control value for eccentricity compensation depending on the playback speed of the disk. Particularly, in a case that the eccentricity has a large influence on the control system, the eccentricity can be compensated when the playback speed of the disk drive is low, but the reliability of the system decreases as the playback speed of the disk increases. Since an increase in the rotational speed of a disk drive system requires an increase in the playback speed of the disk, effective eccentricity compensation is strongly needed. In a case of a high eccentricity-bearing disk, as the playback speed of the disk drive servo system increases, the influence of the eccentricity also increases, and the eccentricity prevents a uniform lead-in operation during the tracking control. To overcome this problem, a maximum limit is set for the playback speed according to the amplitude of the eccentricity in the typical disk drive servo system.
Accordingly, to solve the above problems, it is necessary to determine a control value considering various driving frequencies as well as eccentricity values in order to efficiently compensate for the disk eccentricity. Consequently, a new eccentricity compensation method and apparatus which can effectively adapt themselves to a high rotational speed are desired.